This invention relates to a system for selective detection of chemical substances and measurement of the concentration thereof; and more particularly to a field-effect integrated circuit adapted for detecting and measuring chemical substances, and to a method of using such an integrated circuit (or descrete equivalent thereof) for chemical measurements.
The chemical sensitive field-effect transistor (FET) has, in recent years, provided a valuable technique for measuring and detecting selected chemical properties. The basic chemical sensitive FET, or CHEMFET as it has come to be known (or ISFET--ion sensitive FET), was first disclosed in U.S. Pat. No. 4,020,830 (1977). This basic CHEMFET consists primarily of a semiconductor substrate in which a pair of spaced-apart diffusion regions are placed at an upper surface thereof. A chemical sensitive medium or layer is placed over these spaced-apart diffusion regions and the surface area lying therebetween (known as the gate area), often with an insulating layer between this medium and the surface of the substrate. As the chemical sensitive medium or layer is exposed to chemical substances, a voltage potential (hereinafter referred to as an electrochemical potential for purposes of this application) induces or enhances a conductive channel between the spaced-apart diffusion regions. Thus, when such an electrochemical potential is present, current may flow from one diffusion region through the enhanced channel to the other diffused region. Where spaced-apart regions are used in this manner, the FET is referred to as an "enhancement mode" FET because it will not allow current to flow therethrough unless a voltage is present over the region lying between the diffusion regions.
Another type of FET is the "depletion mode" FET, in which current is allowed to flow between the diffusion regions when no voltage is present over the gate area. The amount of current that flows through the channel is controlled by a voltage which "depletes" the width thereof in a manner that is well known in the art so as to eventually "pinch off" the channel when a potential of a certain value is reached. Depletion mode devices may also be used for CHEMFET applications whenever the electrochemical potential is the potential used to "deplete" the width of the conductive channel.
The initial disclosure of the CHEMFET device in U.S. Pat. No. 4,020,830 has been followed by other publications. See, e.g., P. W. Cheung, D. G. Fleming, W. H. Ko, and M. R. Neuman (editors), "Theory, Design, and Biomedical Applications of Solid State Chemical Sensors", CRC Press, West Palm Beach, Florida (1978), and especially an article therein entitled "Ion-Sensitive Implantable Electrodes Fabricated by Hybrid Technology" by S. S. Yee and M. A. Afromowitz, pp. 81-87. Also see U.S. Pat. No. 4,133,735 which discloses a type of hybrid ion-sensitive electrode in which the ion-sensitive structure is physically separated from the FET.
One of the problems of CHEMFET (or ISFET) devices is an inherent inability to be fully characterized as far as performance characteristics are concerned. That is, heretofore, the only way to apply a potential to the "gate" of the CHEMFET device, thereby enabling the CHEMFET to operate, was to apply an electrochemical voltage thereto. This meant exposing the CHEMFET device to a chemical substance or solution. However, by so doing, it is difficult to separate the chemical effects from the other performance characteristics of the device. For example, the CHEMFET device can be temperature and light dependent; yet it is difficult to separate those variations in the CHEMFET device performance that are attributable to light and temperature changes versus those that are attributable to changes in the chemical properties of the substance to which the device is exposed. Thus, generally speaking, the prior art CHEMFET devices must be used in a highly controlled environment, one in which temperature is held very constant, and where light is either blocked out or is also held at a very constant level. Furthermore, it has not been possible to use the CHEMFET device to characterize the performance of particular chemical sensitive membranes or layers that are used therewith. Whereas, if the complete voltage-current characteristics of a given CHEMFET device were known, then it would be possible to insert thereon a particular chemical sensitive membrane or layer that was to be evaluated, measure the current that flows through the device, and from such a measurement accurately predict how much of an electrochemical voltage the particular chemically sensitive membrane or layer must have produced to allow the measured current to flow.
Another problem with prior art CHEMFET or (ISFET) devices is their high susceptability to damage during manufacture or routine handling due to electrostatic charges.
Still another problem of the prior art CHEMFET (or ISFET) devices is that the chemical sensitive layer or membrane used with the device can be easily contaminated or be incompatable with the semiconductor substrate and diffusion regions. Thus, the materials for the chemically sensitive membrane or layer and the semiconductor materials of the FET device must be carefully selected. This results in a severe limitation as to the range of materials to which the CHEMFET device may be sensitized.
A further problem associated with the prior art enhancement mode CHEMFET devices is that it is not possible to operate them without exposing them to a chemical substance (thereby generating an electrochemical gate voltage that enhances a channel through which current may flow). Thus, these devices cannot be "burned-in", or stabilized, as is common practice with semiconductor devices, unless such a "burn-in" is performed while exposing the chemical sensitive layer to a chemical substance, which exposure is not practical over a long period of time.